Process for preparing biaxially stretched isotropic polyimide film

ABSTRACT

An isotropic, biaxially stretched polyimide film prepared by casting a solution of a polyamic acid precursor in an organic solvent containing a cyclization catalyst and dehydrating agent onto the surface of a support, imidizing the polyamic acid to give a continuous self-supporting gel film containing 5-50% by weight of solids, stretching the gel film in the machine direction (MD) at a ratio of 1.1-1.9, and stretching in the transverse direction (TD) with respect to the machine direction so as to maintain a TD/MD stretch ratio of from 0.9 to 1.3.

This is a division of application Ser. No. 07/841,807, filed Feb. 26,1992 now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a polyimide film for use as a supportfor electrical circuit boards to which a metal layer is either laminatedor deposited and to a process for the manufacture thereof. Specifically,it relates to a biaxially stretched polyimide film having excellentdynamic properties and in-plane isotropy, as well as improveddimensional stability, and a process for the manufacture thereof.

Due to its high heat resistance and good electrical insulationproperties, polyimide film can be used as an electrical insulationmaterial in a broad range of industrial applications which require heatresistance. More particularly, polyimide film can be used as a supportfor an electrical circuit board to which a metal layer is subsequentlylaminated or deposited and wherein an electrical part, such as an ICchip, can be soldered to the metal layer, thereby miniaturizing theelectrical wiring. An electrical circuit board having a polyimide filmsupport can also be folded to form a long continuous electrical circuitboard to such an extent that polyimide film has come to occupy animportant position as a support for electrical insulation applications.However, due to the diversification in the applications of electricalcircuit boards and the increased density in the number of wirings, thepolyimide film requires improvements in dynamic properties, in-planeisotropy, and dimensional stability for use as an electrical insulationsupport. In order to meet these requirements prior art proposals havebeen made based on improving physical properties of the polyimide filmby stretching or by improving dimensional stability by the use of acopolyimide, or the like.

For example, Japanese Patent Application Publication Kokai 63-297029,discloses a process for improving the dynamic properties of an aromaticpolyimide shaped article, comprising cyclizing and imidizing a solutionof an aromatic polyamic acid in an organic polar solvent containing animidization agent, shaping to give a self-supporting shaped articlecontaining 20-87% by weight of volatile solvent and stretching theshaped article at a stretch ratio of at least 1.3 and heat-treating thestretched article at a temperature of at least 150° C. The prior artprocess improves dimensional stability in the stretching direction, butit adversely affects not only the dimensional stability in the directionperpendicular to the stretching direction, but also the dynamic propertyof in-plane anisotropy.

Japanese Patent Application Publication Kokai 44-20878, disclosesstretching a polyamic acid-imide gel film wherein the ratio of imideunits to polyamic acid precursor units is at least 30:70 undirectionallyat 20°-550° C. by at least 5%. However, the prior art reference fails todisclose how to reduce in-plane anisotropy of the film.

U.S. Pat. No. 3,619,461, issued on Nov. 9, 1971, discloses a process forpreparing a polyimide film by immersing a film having a volatilescontent of from 10 to 15% in a volatile liquid and heating andstretching the treated film to increase the orientation in at least onedirection. In the invention process, the polyimide film is not immersedin a volatile liquid prior to stretching.

Japanese Patent Application Publication Kokai 64-16833, Kokai 64-16834,Kokai 1-131241 and Kokai 62-125329, and many similar proposals, disclosethe use of special copolyimides for improving the dimensional stabilityof polyimide films. However, these disclosures fail to disclose a way toreduce in-plane anisotropy of the polyimide film.

The object of the present invention is to provide a polyimide filmhaving improved dimensional stability and dynamic properties andexcellent in-plane isotropy, and to a process for preparing said film.

SUMMARY OF THE INVENTION

According to the present invention there is provided a process formanufacturing an isotropic, biaxially stretched polyimide filmcomprising the steps of:

(a) casting a solution of a polyamic acid precursor of the polyimide inan organic solvent containing a cyclization catalyst and a dehydratingagent onto the surface of a support;

(b) imidizing said polyamic acid to form a continuous self-supportinggel film containing from 5-50% by weight of solids;

(c) stretching said gel film at a stretch ratio of from 1.1-1.9 in themachine direction (MD) wherein the stretching rate is controlled using atension isolation means; and

(d) stretching said gel film in the transverse direction (TD) withrespect to the machine direction (MD) so as to maintain a TD/MD stretchratio of from 0.9 to 1.3.

A further embodiment of the present invention relates to an isotropic,biaxially stretched polyimide film having an in-plane anisotropy indexof not more than 20 and an average in-plane coefficient of thermalexpansion (CTE) at least 10% lower than the unstretched film.

A still further embodiment of the invention relates to a laminate of theisotropic, polyimide film and a metal foil wherein the metal is bondedto the polyimide film either with or without the use of an adhesive.

DETAILED DESCRIPTION OF THE INVENTION

The polyamic acid precursor used to prepare the polyimide of thisinvention is derived from the reaction of an aromatic tetracarboxylicacid and an aromatic diamine and is composed of repeating unitsrepresented by the following Formula (1): ##STR1## wherein R₁ is atetravalent organic group having at least one aromatic ring and whereineach of the two carboxyl groups bonded to R₁ are bonded to carbon atomsof R₁ which are adjacent to carbon atoms bonded to an amide group of thearomatic ring; R₂ is a divalent organic group having at least onearomatic ring and wherein the amino groups are bonded to carbon atoms ofthe R₂ aromatic ring.

The aromatic tetracarboxylic acids used in the polyimide films of theinvention are, for example, pyromellitic acid, 3,3',4,4'-biphenyltetracarboxylic acid, 2,3',3,4'-biphenyl tetracarboxylic acid,3,3',4,4'-benzophenone tetracarboxylic acid, 2,3,6,7-naphthalenetetracarboxylic acid, 2,2-bis(3,4-dicarboxyphenyl)ether,pyridine-2,3,5,6-tetracarboxylic acid, 4,4'-oxydiphthalic acid and theiramide-forming derivatives. Acid anhydrides of these compounds arepreferably used in preparing the polyamic acids.

Aromatic diamines used in the polyimide films of the invention include,for example, paraphenylene diamine, metaphenylene diamine, benzidine,paraxylylene diamine, 4,4'-diaminodiphenylether,4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylsulfone,3,3'-dimethyl-4,4'-diaminodiphenylmethane, 1,5-diaminonaphthalene,3,3'-dimethoxybenzidine, 1,4-bis(3-methyl-5-aminophenyl)benzene,1,2-bis(4-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene,1,2-bis(3-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene,1,4-bis(4-aminophenoxy)benzene, and their amide-forming derivatives.

A particularly preferred polyimide film of the invention is derived frompyromellitic dianhydride and 4,4'-diaminodiphenylether.

Aromatic tetracarboxylic acids and aromatic diamines particularlysuitable for preparing a polyamic acid precursor for manufacturing thepolyimide film according to the process of this invention, also includecombinations of pyromellitic acid dianhydride and4,4'-diaminodiphenylether; and 3,3',4,4'-biphenyltetracarboxylic aciddianhydride and paraphenylene diamine. Particularly preferred herein arepolyimide films derived from 30 to 50 mole % of3,3',4,4'-biphenyltetracarboxylic dianhydride, 50 to 70 mole % ofpyromellitic dianhydride, 60 to 80 mole % of paraphenylene diamine, and20 to 40 mole % of 4,4'-diaminodiphenylether. Gel films prepared frompolyamic acids generated from such combinations can satisfactorilyattain the effect of this invention, i.e. they can be readily biaxiallystretched to TD/MD stretch ratios ranging from 0.9 to 1.3, providingpolyimide films having excellent in-plane isotropy and low coefficientsof thermal expansion.

Organic solvents which can be used in this invention are specifically,for example, organic polar amide solvents such as N,N-dimethylformamide,N,N-dimethylacetamide, N-methyl-2-pyrrolidone, and the like. Theseorganic solvents may be used singly or in mixtures of two or more, orelse in combination with nonsolvents such as benzene, toluene, andxylene. The solutions of the polyamic acid in organic solvents used inthis invention contain from 5-50% by weight, preferably 10-30% byweight, of polymer solids and have a Brookfield viscosity at 20° C. of100-20,000 poises, preferably 1,000-10,000 poises and form stablesolutions. The polyamic acid in the organic solvent solution may bepartially imidated. A small amount of an inorganic additive forimparting slip and handling properties to the polyimide film may also beincluded in the polyamic acid solution.

According to the invention process, at least one aromatictetracarboxylic acid component and at least one aromatic diaminecomponent are polymerized at an approximately equimolar ratio with onecomponent being not more than 10 mole %, preferably not more than 5 mole%, in excess over the other component. The polymerization reaction iscarried out continuously for 10 minutes to 30 hours at temperaturesranging from 0°-80° C. in the organic solvent with agitation and/ormixing. Optionally, the polymerization reaction can be carried out byadding one or more of the components in portions. Although there is noparticular limitation as to the sequence of adding the two reactioncomponents, it is preferred to add the aromatic tetracarboxylic acidcomponent to a solution of the aromatic diamine component. Vacuumdefoaming the polymerization reaction mixture can be used to prepare astable solution of a polyamic acid in the organic solvent. A smallamount of a terminal-blocking agent may also be added to the aromaticdiamine prior to the polymerization reaction in order to regulate thereaction rate of the polymerization reaction.

The cyclization catalysts used in this invention process are typically,for example, aliphatic tertiary amines such as trimethylamine,triethylene diamine, and the like, and heterocyclic tertiary amines suchas isoquinoline, pyridine, beta-picoline, and the like. Preferably, atleast one amine is a heterocyclic tertiary amine.

The dehydrating agents used in this invention process are typically, forexample, aliphatic carboxylic acid anhydrides, such as acetic anhydride,propionic anhydride, butyric anhydride, and the like, and aromaticcarboxylic acid anhydrides, such as benzoic anhydride, and the like.Acetic anhydride and/or benzoic anhydride are preferred.

The amounts of the cyclization catalyst and the dehydrating agent usedwith respect to the polyamic acid are preferably regulated according tothe following Equations (1) and (2): ##EQU1## Optionally, a gel-retardersuch as acetyl acetone, or the like, may also be included.

Typical processes for manufacturing polyimide film from a solution of apolyamic acid in an organic solvent include a thermal cyclizationprocess, which comprises casting a solution of a polyamic acid in anorganic solvent containing no cyclization catalyst or dehydrating agentfrom a slit die onto the surface of a support to form a film, heatingand drying to form a self-supporting film on the support, peeling thefilm from the support, and further drying/heat treating the film at hightemperatures to effect imidation. Alternatively, a chemical cyclizationprocess can be used which comprises casting a solution of a polyamicacid in an organic solvent containing a cyclization catalyst and adehydrating agent from a slit die onto the surface of a support,partially imidating on the support to give a self-supporting gel film,peeling the film from the support, and drying and heat treating toeffect imidation.

The thermal cyclization process is advantageous in not requiring anyapparatus for incorporating the cyclization catalyst or dehydratingagent, but requires a long heating and drying time to generate aself-supporting film, which is subsequently peeled from the support. Thefilm also contains too high of a ratio of polymer solids to allow stablefilm stretching. Therefore, the thermal process is not suitable for usein this invention.

The chemical cyclization process, although requiring an apparatus forincorporating the cyclization catalyst and dehydration agent in thesolution of the polyamic acid in an organic solvent, is a preferredprocess for manufacturing polyimide film according to this invention.The chemical process provides a self-supporting gel film within a shorttime, provides a low ratio of solids in the gel film peeled from thesupport, and permits stretching to the preferred stretch ratio. Aprocess for manufacturing a polyimide film closely resembling a thermalcyclization process by decreasing the concentration of the cyclizationcatalyst and dehydrating agent may be considered as a chemicalcyclization process, since it still contains some cyclization catalystand dehydrating agent.

The methods for incorporating the cyclization catalyst and thedehydrating agent into the solution of a polyamic acid in the organicsolvent include a process of mixing the solution of a polyamic acid inan organic solvent, cyclization catalyst, and dehydrating agent in arotary mixer; a process for feeding the solution of a polyamic acid inan organic solvent to a static mixer while introducing the cyclizationcatalyst and dehydrating agent immediately before the static mixer; aprocess for casting the solution of a polyamic acid in an organicsolvent onto a support, followed by contacting with the cyclizationcatalyst and dehydration agent, and the like. However, in terms of theconcentrations of the cyclization catalyst and dehydration agent used,as well as their uniformity, it is preferred to mix the cyclizationcatalyst, dehydrating agent, and the solution of a polyamic acid in anorganic solvent and feed the liquid mixture through a slit die. In orderto insure formation of a stable liquid mixture, said mixed liquid musthave its solids concentration and temperature controlled to maintain aviscosity of 100-10,000 poises. Cyclization of the polyamic acid canalso result in a viscosity which is too high to allow extrusion of theliquid mixture through the slit die, therefore, the solution should bemaintained at a low temperature (for example, -10° C.).

The liquid mixture is cast from the slit die in the form of a continuousfilm onto the surface of a heated support. The polyamic acid undergoes aring closure reaction on the support to give a self-supporting gel filmwhich is then peeled from the support. The support may be a metalrotating drum or endless belt, the temperature of which is regulated bya liquid or gas heating medium and/or radiant heat from an electricalheater, or the like.

The gel film undergoes a ring closure reaction upon heating to 30°-200°C., preferably 40°-150° C., from the heat received from the supportand/or from an external heat source, such as hot air, an electricalheater, or the like, to form a self-supporting film which is peeled fromthe support. The rapid heating of a polyamic acid film which has notfully progressed in the ring closure reaction fails to give aself-supporting gel film, so that the heating temperature must becarefully controlled.

The gel film peeled from the support is first stretched in the machinedirection (MD) while controlling the stretching rate using a tensionisolation means. The stretching is carried out at a stretch ratio offrom 1.1-1.9, preferably 1.1-1.6, at temperatures not higher than 150°C. The stretching rate of the tension isolation means is regulated bythe drive source and a speed regulator. The tension isolation means musthave a gripping strength sufficient to regulate the gel film movingrate. Examples of tension isolation means include nip rolls comprising acombination of metal and rubber rolls and/or vacuum suction rolls, wherethe number of rotating rolls is selected depending upon each roll'sgripping force. The gripping force required is from 50-1,000 kg/m withrespect to the gel film width. A stretch ratio in the machine directionof the gel film of less than 1.1 gives a small stretching effect andfails to improve dynamic properties. As the stretch ratio in the machinedirection is increased, the stretching effect is increased, therebyimproving the dynamic properties and dimensional stability in themachine direction. However, because the range of the subsequent stretchratio in the transverse direction is limited by gel film breakdowns, thestretch ratio in the machine direction must be from 1.1-1.9, preferably1.1-1.6, to improve the in-plane isotropy, which is one of theobjectives of this invention.

The gel film stretched in the machine direction is subsequentlyintroduced into a tenter frame where it is gripped at both transverseedges. Various means may be employed to grip the film, including varioustypes of pins, clips, clamps, and rollers. Most preferably, gripping ofthe film for stretching in the transverse direction is obtained by clipswhich are mounted on an endless chain on each side of the film. The gelfilm is then stretched in the transverse direction due to outwardmovement of the tenter clips, the volatile organic solvent is removed byvaporization and the film is heat treated to give a biaxially stretchedpolyimide film. The transverse stretching is carried out at temperaturesnot more than 400° C., preferably not more than 350° C., at a TD/MDstretch ratio, i.e. a stretch ratio defined by the following Equation(3), of 0.9-1.3, preferably 1.0-1.3. ##EQU2## The TD/MD stretch ratio isa very critical factor for improving the in-plane isotropy of the film,which is one of the objectives of this invention. A TD/MD stretch ratioof less than 0.9 results in a stretching effect in the machine directionwhich is too high and a TD/MD stretch ratio exceeding 1.3 results in astretching effect too high in the transverse direction, thereby causingthe film in-plane isotropy to deviate from the preferred range.

In a further embodiment of the invention process, rather than stretchingthe gel film in the moving direction (MD) followed by stretching in thetransverse direction (TD), the gel film can be stretched simultaneouslyin both the moving direction (MD) and the transverse direction (TD).

The stretchability of the gel film is strongly dependent upon its solidsconcentration. As the solids concentration approaches 60% by weight,stretching of the gel film becomes more difficult. Such a film ifstretched to a stretch ratio of 1.05 in the moving direction cannotundergo transverse stretching due to the breakdown of the gel film.Therefore, the gel film peeled from the support must have a solidsconcentration of not more than 50% by weight. In order to generate aself-supporting gel film, the solids concentration should preferably be5-50% by weight.

The gel film in the tenter frame is dried and heat treated by means ofhot air and/or radiant heat from an electrical heater, or the like, at adrying temperature of 100°-400° C. and a heat treatment temperature of350°-500° C. Too rapid heating of the gel film results in the rapidremoval of volatile matter contained in the film, generates spongydefects on the resultant biaxially stretched polyimide film surface andresults in a loss of film smoothness.

The resultant biaxially stretched polyimide film thus prepared has itsmolecular chains uniformly oriented in the plane of the film whichresults in a lowering of the CTE according to Equation (4). ##EQU3##

The resultant biaxially stretched polyimide film also has an in-planeanisotropy index defined by the following Equation (5) of not more than20, thus showing excellent in-plane isotropy. ##EQU4##

The biaxially stretched polyimide film prepared by the process of thisinvention is in-plane oriented, has a coefficient of thermal expansion10% less than unstretched film, and has an in-plane anisotropy index ofnot more than 20 over the entire film surface, which minimizes theextent of curling of flexible metal-clad boards.

The present invention improves the dynamic properties of a polyimidefilm and, simultaneously, its in-plane isotropy and dimensionalstability. Moreover, the polyimide film can be readily adapted todiversified electrical circuit board applications requiring increasedwiring densities by laminating or depositing metal foil to the polyimidefilm.

The following methods were used to measure the properties of thepolyimide films prepared according to the process of the invention.

(1) In-plane Anisotropy Index

Anisotropy was measured using a Sonic Sheet Tester SST-250 Modelmanufactured by the Nomura Shoji Company. A specimen was prepared bystacking six sheets of 25 micron film and accurately cutting out a piecemeasuring 250 mm in the machine direction and 170 mm in the transversedirection. The specimens representing the center of the film weresampled from the center in the transverse direction, while edgespecimens were sampled from a center location 100 mm from the edge.

The sonic wave propagation rate through the film samples was measured at10° intervals. The data was correlated in terms of a second order curveso as to obtain the distribution of orientation throughout the entirecircumference, from which the maximum orientation angle, minimumorientation angle, and sonic wave propagation rates at the maximum andminimum orientation angles were determined.

The in-plane anisotropy index (AI value) was calculated from Equation(5) based on the sonic wave propagation rate in the direction of themaximum orientation angle (peak value max), and the rate of sonicpropagation in the direction of the minimum orientation angle (peakvalue min.).

(2) Coefficient of Thermal Expansion (CTE)

CTE was measured using an Orton Dilatometer equipped with a BarberColman programmer-controller.

A characteristic expansion curve was obtained by heating a rigid filmsample to 300° C. at a heating rate of 5°-20° C./min, cooling the sampleto room temperature and then reheating the sample to 300° C. The CTE isthe expansion of the film in parts per million per °C. measured between150° and 250° C. upon reheating the sample. The rigid sample of 25micron film was made by winding a 2.5×13 cm strip into a cylinderapproximately 4 mm in diameter and maintaining the cylindrical shapewith wire ties.

The average coefficient of thermal expansion was obtained from thecoefficients of thermal expansion measured in the direction of theminimum and maximum orientation angles, i.e., average CTE=minimum(CTE)+maximum (CTE)/2

(3) Curling

A specimen 250 mm long in the machine direction and 50 mm long in thetransverse direction was cut out from a flexible polyimide copper cladlaminate and was placed on a table to measure its curling height. Theextent of curling was evaluated by measuring the heights at the fourcorners of the flexible laminate, h₁, h₂, h₃ and h₄ and calculating theextent of curling by Equation (6). ##EQU5##

The extent of curling was rated as follows:

Small: less than 10 mm

Medium: 10 mm or more, but less than 30 mm

Large: 30 mm or more.

The significance of the extent of curling is an empirical measure andindicates the ease of incorporating a flexible printed circuit baseboardinto an electrical device:

Small: Easily incorporated

Medium: Can be incorporated with some effort

Large: Incorporated sometimes but with difficulty.

The advantageous properties of this invention can be observed byreference to the following examples which illustrate, but do not limit,the invention. All parts and percentages are by weight unless otherwiseindicated.

Example 1

To a solution of 20.024 kg (0.1 kmol) of 4,4'-diaminodiphenylether in190.6 kg of dry N,N-dimethylacetamide was added while agitating at 20°C. in small portions 21.812 kg (0.1 kmol) of purified powderypyromellitic dianhydride, followed by continuous stirring for one hourto give a clear polyamic acid solution. The solution had a viscosity of3500 poises at 20° C. The polyamic acid solution was mixed with 2.5moles of acetic anhydride with respect to the polyamic acid units and2.0 moles of pyridine with respect to the polyamic acid units, whilecooling to give a solution of the polyamic acid in the organic solvent.The resultant solution of the polyamic acid in the organic solvent wasmetered through a slit die and cast onto a metal drum at 90° C. to givea self-supporting gel film. The resultant gel film contained 21% byweight solids. The gel film was peeled from the metal drum and wasstretched in the machine direction (MD) at a temperature of 65° C.between two sets of nip rolls, consisting of metal and silicone rubberrolls, followed by feeding the film to a tenter frame. The stretch ratioin the machine direction, that is, the ratios of the speeds of the metaldrum, each set of nip rolls, and the tenter were adjusted to 1.12 forthe ratio of the speed of the first set of nip rolls with respect to thespeed of the metal drum, 1.23 for the second set of nip rolls, and 1.39for the tenter. The film was stretched to a stretch ratio of 1.61 in thetransverse direction in the tenter (TD/MD stretch ratio was 1.16), driedfor 40 seconds at a temperature of 260° C., heat treated for one minuteat 430° C., cooled for 30 seconds while allowing the film to relax, andcutting off the edges of the film to give a 1997 mm wide and 25 micronthick biaxially stretched polyimide film. The film had an averagecoefficient of thermal expansion of 27.5 ppm/°C. It also had an in-planeanisotropy index at the edge of 8, and an in-plane anisotropy index atthe center of 7.

The polyimide film was subsequently coated with a polyester/epoxy typeadhesive using a roll coater, followed by drying at 160° C. A piece ofelectrolytic copper foil was pressure-laminated onto the adhesive coatedside of the film at 130° C., and cured for 24 hours to give a flexiblepolyimide copper clad sheet. The extent of curling of the copper cladsheet was rated as small.

Example 2

Example 1 was repeated except that the ratio of the speed of the firstset of nip rolls to that of the metal drum was 1.12, that of the secondset of nip rolls was 1.23, and that of the tenter was 1.38, and thestretch ratio was 1.58 in the transverse direction (TD/MD stretch ratiowas 1.14), to give a 2130 mm wide and 25 micron thick biaxiallystretched polyimide film.

The film had a CTE of 27.5 ppm/°C. and an in-plane anisotropy index atthe edge of 4, and an in-plane anisotropy index at the center of 4. Aflexible polyimide copper clad sheet was obtained in a manner similar tothat of Example 1. The curl rating of the sheet was small.

Example 3

Example 1 was repeated except that the ratio of the speed of the firstset of nip rolls to that of the metal drum was 1.09, that of the secondset of nip rolls was 1.14, and that of the tenter was 1.22, and thestretch ratio was 1.30 in the transverse direction (TD/MD stretch ratiowas 1.07) to give a 1898 mm wide and 75 micron thick biaxially stretchedpolyimide film.

The film had a CTE in the center of 31 ppm/°C., and an in-planeanisotropy index at the edge of 15 and an in-plane anisotropy index atthe center of 10. A flexible polyimide copper clad sheet was obtained ina manner similar to that of Example 1. The sheet had a curling rating ofmedium.

Comparative Example 1

This example shows that film prepared using low TD/MD stretch ratios andwithout using nip rolls had a significantly higher anisotropy index atboth the edge and the center and a 20% higher CTE than a film preparedaccording to Examples 1 and 2 of the invention.

Example 1 was repeated except that no nip rolls were used, the ratio ofthe speed of the tenter to that of the metal drum was 1.16 and thestretch ratio was 1.30 in the transverse direction (TD/MD ratio was1.12) to give a 2000 mm wide and 25 micron thick biaxially stretchedpolyimide film. The film had an average CTE of 35 ppm/°C., an in-planeanisotropy index at the edge of 41, and an in-plane anisotropy index atthe center of 10. A flexible polyimide copper clad sheet was prepared asdescribed in Example 1 and had a curling rating of small for the centerand large at the edge.

Comparative Example 2

This example shows that film prepared using lower TD/MD stretch ratiosand without using nip rolls has a significantly higher anisotropy indexat the edge than a film prepared according to Example 3 of theinvention.

Example 3 was repeated except that nip rolls were not used, the ratio ofthe speed of the tenter to that of the metal drum was 1.14, and thestretch ratio was 1.25 in the transverse direction (TD/MD ratio was1.10) to give a 2085 mm wide and 75 micron thick biaxially stretchedpolyimide film.

The film had an in-plane anisotropy index at the edge of 30 and anin-plane anisotropy index at the center of 5. The average CTE of thefilm was 35 ppm/°C. A flexible copper clad sheet was obtained in amanner similar to that of Example 1. The copper clad sheet had a curlingrating of small at the center and large at the edge.

Comparative Example 3

This example shows that the CTE of film prepared from gel film which wasnot stretched, i.e., MDX=TDX=1.0, and without using nip rolls, had amuch higher CTE when compared to film prepared according to Examples 1and 2 of the invention.

Example 1 was repeated except that the gel film was placed on a pintenter frame to control both MD and TD stretching at 1.0. The film wastoo small to measure the anisotropy index. The average CTE was 40ppm/°C.

The results obtained for Examples 1 to 3 and Comparative Examples 1 to 3are summarized in Table 1.

                                      TABLE 1                                     __________________________________________________________________________              Examples       Comparative Examples                                           1    2   3     1   2    3                                           __________________________________________________________________________    Nip Rolls Used Used                                                                              Used  Not Not  Not                                                                  Used                                                                              Used Used                                        Stretch Ratio                                                                           1.39 1.38                                                                              1.22  1.16                                                                              1.14 1.0                                         in the Moving                                                                 Direction (MD)                                                                Stretch Ratio                                                                           1.61 1.58                                                                              1.30  1.30                                                                              1.25 1.0                                         in the Transverse                                                             Direction (TV)                                                                Ratio of Stretch                                                                        1.16 1.14                                                                              1.07  1.12                                                                              1.10 1.0                                         Ratios (TD/MD)                                                                Anisotropy Index                                                                        7    4   10    10  5    --                                          (Center)                                                                      Anisotropy Index                                                                        8    4   15    41  30   --                                          (Edge)                                                                        Film Thickness                                                                          25.0 25.0                                                                              75.0  25.0                                                                              75.0 25.0                                        (microns)                                                                     Average Coefficient                                                                     27.5 27.5                                                                              31.0  35.0                                                                              35.0 40.0                                        of Thermal Expan-                                                             sion (ppm/°C.)                                                         Extent of Curling                                                                       Small                                                                              Small                                                                             Medium                                                                              Small                                                                             Small                                                                              --                                                                   to  to                                                                        Large                                                                             Large                                            __________________________________________________________________________

What is claimed is:
 1. A process for the manufacture of an isotropic,biaxially stretched polyimide film, comprising the steps of:(a) castinga solution of a polyamic acid precursor of the polyimide in an organicsolvent containing a cyclization catalyst and a dehydrating agent ontothe surface of a support; (b) imidizing said polyamic acid to form acontinuous self-supporting gel film containing from 5 to 50% by weightof solids on the surface of said support; (c) stretching said gel filmat a stretch ratio of from 1.1 to 1.9 in the machine direction (MD)wherein the stretching rate is controlled using a tension isolationmeans; and (d) stretching said gel film in the transverse direction (TD)with respect to the machine direction (MD) so as to maintain a TD/MDstretch ratio of from 0.9 to 1.3; and (e) heat-treating said stretchedgel film at a temperature of from 350° to 500° C. under restraint tocompletely imidize said polyamic acid to said polyimide, wherein saidisotropic polyimide film has an in-plane anisotropy index of not morethan
 20. 2. The process for the manufacture of an isotropic, biaxiallystretched polyimide film of claim 1 wherein said gel film of step (c) isstretched simultaneously in both the machine (MD) and transverse (TD)directions.
 3. The process of claim 1 wherein the polyimide is derivedfrom the reaction of at least one aromatic tetracarboxylic aciddianhydride and at least one aromatic diamine.
 4. The process of claim 3wherein the aromatic tetracarboxylic acid dianhydride is pyromelliticdianhydride and the aromatic diamine is 4,4'-diaminodiphenyl ether. 5.The process of claim 3 wherein the aromatic tetracarboxylic aciddianhydride comprises pyromellitic dianhydride and3,3',4,4'-biphenyltetracarboxylic dianhydride and the aromatic diaminecomprises 4,4'-diaminodiphenyl ether and paraphenylene diamine.
 6. Theprocess of claim 1 wherein the gel film of step (c) is stretched in themachine direction at a stretch ratio of 1.1 to 1.6.
 7. The process ofclaim 6 wherein the gel film of step (c) is stretched in the machinedirection at a temperature of less than 150° C.
 8. The process of claim1 wherein the gel film of step (d) is stretched in the transversedirection with respect to the machine direction so as to maintain aTD/MD stretch ratio of from 1.0 to 1.3.
 9. The process of claim 8wherein the gel film from step (d) is stretched in the transversedirection at a temperature of less than 400° C.
 10. The process of claim1 wherein the tension isolation means in step (c) comprises nip rollshaving a film gripping force of from 50 to 1000 Kg/m.
 11. The process ofclaim 1 wherein the tension isolation means in step (c) comprises vacuumrolls having a film gripping force of from 50 to 1000 Kg/m.